The present invention relates to a new polyethylene terephthalate raw material, to a process for its manufacture, and to its use.
The preparation of polyethylene terephthalate by polycondensation, with the use of catalysts, such as antimony oxides, has been well known for a long time and need not be described in detail. Furthermore, it is well known in the prior art to add catalysts, e.g., calcium, lithium, manganese or zinc salts, during the interchange of ester radicals which precedes polycondensation. The catalysts are mainly chosen with the objective in mind of achieving an economical preparation of the raw material, faster reaction times, favorable color shades, and the like.
In addition it is known from British Pat. No. 951,768 to add pigments, such as calcium carbonate, calcium silicates, glass, titanium dioxide, or silicium dioxide to polymers, e.g., polyethylene terephthalate, for the manufacture of films, with the quantities added ranging from 1 to 25 percent by weight. The particle size of the pigments may range from 0.3 to 20 .mu.m. Depending on their type, grain size, and quantity, the pigments added change the physical properties of the resulting film, which may become either matte or transparent in appearance. In some cases, by this addition, the surface of the film is modified in such a way that it is possible to write upon it.
Polyethylene terephthalates prepared with the aid of esterification catalysts possess certain disadvantages, however. After being melted for the preparation of shaped articles, such as films or filaments, some compositions are not easily filterable, or the useful lives of the filters are relatively short, or they tend to form deposits on the walls of the condensation vessel so that coarse impurities may get into the raw material. Shaped articles produced from such raw materials, e.g., films, frequently show spots, as a result of which their usability, for example as condensor films, is considerably impaired.
Furthermore, the depth of roughness of the film and its surface waviness are frequently unsatisfactory, because it is desirable to make the surfaces as even as possible. In this regard, the slip of the film also plays an important role.